U-value of a SIP roof (structural insulated panels)

A pitched roof of 172mm structural insulated panels — a 142mm polyurethane core (λ 0.024) between 15mm OSB skins, with 4% timber at the joints, eaves and ridge — tiled on counter-battens and lined with plasterboard on a 25mm service void has a U-value of about 0.17 W/m²K: short of the 0.16 Approved Document L sets for the roof of a new dwelling and the 0.15 for a new roof on an extension. The timber fraction is the variable: with no timber the same panel gives 0.15, the figure in Kingspan’s low-pitched roof note (which states no timber fraction), and with 8% 0.19. 25mm of PIR in the service zone takes it to 0.14, and 50mm to 0.13.

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The build-up (outside → inside)

Tiles on battens and 25 × 50mm counter-battens: ventilated, not counted
Vapour-permeable underlay on the panel
15mm OSB/3 outer skin
142mm polyurethane core (Kingspan TEK, λ 0.024), 4% timber at the joints, eaves and ridge
15mm OSB/3 inner skin
25mm service void on battens
12.5mm plasterboard

Panel core at Kingspan’s declared λ 0.024 (TEK Building System specification manual; the 2010 BBA certificate used 0.023), with 15mm OSB/3 skins at λ 0.13. The timber is entered as a bridge through the core. U-Monkey applies BR 443 §4.8.1’s level 0 to a bonded SIP core, and level 1 once a separate insulation layer is added in the service zone. The service-zone rows use Kingspan Thermapitch TP10 (λ 0.022), whose foil facing has a declared emissivity of 0.05 (BBA 14/5133), so the 25mm void behind it counts as a low-emissivity air space (BS EN ISO 6946 Annex D), as U-Monkey applies automatically. The tiles and counter-battens sit outside a ventilated batten space and are not counted.

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
142mm panel (112mm core), no timber	≈ 0.19	Kingspan TEK’s certificate: 0.18–0.20 for its typical roof
142mm panel, 4% timber	≈ 0.21
172mm panel (142mm core), no timber	≈ 0.15	Kingspan’s low-pitch note: 0.15
172mm panel, 4% timber	≈ 0.17	As pre-filled
172mm panel, 8% timber (solid splines, eaves and ridge plates)	≈ 0.19
172mm panel + 25mm PIR in the service zone, 4% timber	≈ 0.14	Meets 0.15 (0.144); Kingspan’s note: 0.13
172mm panel + 50mm PIR in the service zone, 4% timber	≈ 0.13	Kingspan’s note: 0.11
Hemsec 200mm panel (178mm core, λ 0.028), 4% timber	≈ 0.16	Hemsec’s data sheet: 0.15 for the panel alone

Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and the air-gap correction included — exactly as the pre-filled calculator works them out, with any pass or fail judged on the unrounded U-value. Indicative for the generic build-up shown: your products, thicknesses and (for floors) footprint will move the number, and that's what the calculator is for.

What changes the number

A SIP roof’s U-value turns on how much timber runs through the core. BR 443 says the panel’s resistance is calculated separately, with the bridging fraction set by how the panel is made, and gives no SIP default: Kingspan’s TEK certificate takes 0–1% timber for its roof (0.18–0.20 for the 142mm panel), U-Monkey’s SIP wall page uses 4%, and solid timber splines at every joint, with eaves and ridge plates, push it higher (the 8% row). Each extra 1% costs about 0.005. Panel depth matters next — 142mm to 172mm takes the same timber fraction from 0.21 to 0.17 — and then the service zone: 25mm of PIR on the inner skin, behind the battens, brings the 172mm panel to 0.14, and 50mm to 0.13 — the board’s foil facing also makes the 25mm void behind it a low-emissivity air space; Kingspan’s low-pitched roof note gives 0.13 and 0.11 for those, with no timber fraction stated. The bonded core has no air gaps to correct for (BR 443 §4.8.1, level 0, which U-Monkey applies to a SIP core); once a separate board is added in the service zone, behind the inner skin, U-Monkey brings back level 1 — a cautious reading of BR 443’s level 0 for double-layer insulation. Other panels differ by core: Hemsec’s polyurethane cores are λ 0.028–0.030 by thickness, its 200mm panel 0.15 on its own. Approved Document L sets 0.16 for the roof of a new dwelling (Table 4.1) and 0.15 for a new roof on an existing one (Table 4.2), and the notional dwelling assumes 0.11, which even the 50mm service-zone layer only approaches (0.12 with no timber).

Condensation risk in a SIP roof

U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in any month for the pre-filled roof, but does not give it a pass: as a warm pitched roof with no separate air and vapour control layer, it falls outside the cases BS 5250:2021 covers, and a calculation cannot see the air leakage that guidance guards against. Kingspan’s BBA assessment found no vapour control layer necessary for its typical tiled roof under a very vapour-open underlay; its low-pitched roof note, on the other hand, asks for a polythene vapour control layer and a humidity class no higher than 3.

Either way, the joints are what matter in a SIP roof: the panels resist vapour well, but air leaking through unsealed joints carries moisture straight to the cold outer skin. Seal every joint as the system’s details show, and follow the panel maker’s certified build-up.

Check this build-up for condensation — free

Opens pre-filled. The condensation verdict sits just under the U-value — tap it for the full month-by-month check, the surface-mould (f_Rsi) check included. If it isn’t a pass, How to fix it tests the usual remedies on your build-up — a vapour control layer, a vapour-open underlay, a ventilated void, a warm deck — and applies the one you pick in a tap, with Undo.

Need it in writing for Building Control? The full condensation risk analysis report is £25 +VAT (£30 inc VAT), or included on Monkey Pro Unlimited — see an example report (PDF), or read what building control expects the analysis to show.

It is a screening to BS EN ISO 13788 (the Glaser method), honestly labelled as one: it does not model air leakage, rain or moisture stored in materials, so for high-risk or heritage constructions a full hygrothermal simulation (BS EN 15026) is the right next step. Figures above use the app’s defaults — Manchester (Ringway) climate, humidity class 3 (dwellings with high or unknown occupancy), a 1-in-10 design year and, on a roof, the surface taken 2 K below the air for night-sky cooling — so pick your nearest weather station for your own build-up: the result can change from one station to another.

Next: check the junctions

A U-value covers the flat area of an element — but heat also escapes where elements meet, and at modern U-values those junctions can be 20–30% of a dwelling's fabric heat loss. Now you know your U-values, model the junctions with the free ΨMonkey psi value calculator (no subscription; PDF reports £25 +VAT a junction):

Or read thermal bridging explained to see how junction Ψ-values feed your SAP calculation.

Frequently asked questions

What is the U-value of a SIP roof (structural insulated panels)?

What timber fraction should I use for a SIP roof?

The one your panel system actually has — BR 443 sets the fraction “according to the construction of the panel” and gives no default. Panels joined with insulated cassette joints carry very little timber (Kingspan’s TEK certificate assumes 0–1% for its roof); solid timber splines at every joint, plus eaves and ridge plates and trimmers round roof windows, add a good deal more. On a 172mm panel, no timber gives 0.15 W/m²K and 8% gives 0.19, so take the figure from the system’s own calculation, and enter it in U-Monkey as the bridge through the core.

Is this calculator free?

Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard), the condensation-risk screening check and the thermal-mass figures (decrement factor, time lag, κ) are all free in your browser, and the Building Control-ready PDF report is free too. No sign-up.

Does it follow the official calculation method?

Yes — the combined method of BS EN ISO 6946:2017 with BR 443 conventions, including timber bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and air-gap corrections, BS EN ISO 13370 for ground floors, suspended floors and basements, and ISO 13788 for the condensation screening check.

Other common build-ups

Roofs

Walls

Floors

Different build-up? See the full roof U-value calculator, or start from scratch in the main U-value calculator.

Need the wider compliance picture? U-values feed straight into SAP calculations — or let our New Build Compliance Pack handle the lot. Background reading: what U-values actually are, and why two calculators can give different answers for the same wall.